The present invention relates to control of the operation of rotary positive displacement fluid pumps and, more particularly, to control of the operation of variable flow rate rotary positive displacement fluid pumps.
Rotary positive displacement fluid pumps are operated relatively simply by connecting the pump rotor to a source of mechanical torque, typically an electric motor or an engine, either directly or through some kind of a mechanical interconnection arrangement such as gears. The resulting rotation rate of the pump rotor determines the volume of the pump output fluid flow, and so a substantially constant engine rotation rate leads to a substantially constant pump output fluid flow rate. Such a pumping arrangement for providing fuel to gas turbine engines has been common in the past with the mechanical torque source having been a shaft extending to the pump rotor from the accessory gearbox of the gas turbine engine receiving the flow of fuel from the pump.
However, gas turbine engines used in aircraft operate over a rather large range of engine rotation rates in rotating the gears in its accessory gearbox, and so often, at greater engine rotation rates, such engines cause the fuel supplied thereto by the corresponding fuel pump formed by a rotary positive displacement fluid pump to be in quantities that are substantially in excess of that needed to fuel operation of the engine. As a result, the excess fuel is typically recirculated back to a location in the fuel delivery system ahead of the fuel pump inlet. Such pressurizing of the fuel by the pump, and then the subsequent depressurizing thereof in returning that excess fuel to ahead of the pump inlet, more or less continually over a span of time causes the fuel to become significantly heated. The ensuing large fuel temperature increases have various detrimental effects with respect to operation of the turbine engine.
Avoiding much of such heating has been accomplished by substituting variable flow rate control of rotary positive displacement fluid pumps in such a manner as make the pump output flow rate much less dependent on the rotation rate of the pump rotor, and so much less dependent on the rotation rates in the accessory gearbox of the gas turbine engines to which they are correspondingly supplying fuel. Typically, a vane pump is used as a rotary positive. displacement fluid pump serving as the fuel pump in which the pump displacement can be varied, as well as the rotation rate of the pump rotor, to together determine the pump output flow rate. The variation of pump displacement can be achieved, for example, by changing the relative position of typically, a cam and the pump rotor within which that pump rotor is mounted off center to move cycloidally with respect thereto. Alternatively, a reciprocating piston pump with a variable position swash plate can be used to provide a variable rotation rate, variable displacement pump for a fuel pump. Relatively elaborate hydraulic pump control systems use the fuel as the “working fluid” in the control system, i.e. a “fueldraulics” control system, as well as that fluid being supplied by the pump and its control system to the corresponding gas turbine engine to be the engine fuel therefor.
Such a pump displacement (and so pump output flow rate) control system and vane pump arrangement, 10, is shown in a schematic representation block diagram in FIG. 1. A vane pump, 11, is shown with an inlet, 12, to which fuel is supplied through an inlet pressure source, 12′, and the pump is also is connected to a shaft, 13, to which torque for rotating the pump rotor is supplied as a result of this shaft extending to the accessory gearbox of a gas turbine engine (not shown). Vane pump 11 has an outlet, 14, from which fuel pressurized by this pump is provided both to a pressured fluid conduit, 15, to a metering valve, 16, and an excess flow. bypass conduit, 17. Valve 16, in cooperation with a regulator, 18, operates to provide the desired rate of fuel flow at its outlet, 19, to the gas turbine engine combustor (not shown) in compensating for the loss of fuel from the output of vane pump 11 through pressured fluid conduit 15. The overflow fuel is sent to regulator 18, through excess flow bypass conduit 17, and recirculated to vane pump 11 through a recirculation conduit, 20, carrying the fuel to inlet 12. The overflow amount is determined by regulator 18 determining the differential pressure across metering valve 16 through a pair of differential pressure sensing conduits, 21.
The main control for the displacement of pump 11 is provided typically as part of the engine electronic controller, 22, which receives both commands to change thrust and various sensor inputs at its input, 23. Controller 22 through an electrical interconnection, 24, operates an electrohydraulic servovalve, 25, with excess fluid therein returned to vane pump 11 through an excess fluid return conduit, 25′, carrying the fuel to recirculation conduit 20. Electrohydraulic servovalve 25 in turn operates a hydraulic actuator, 26, through adding fluid thereto and removing fluid therefrom in control conduits, 27, to thereby force its output ram shaft, 28, to the left or right in FIG. 1. Such motion of output ram shaft 28 thereby alters the position of the typical cycloidal motion cam in vane pump 11 so as to either increase or decrease the fluid displacement of that pump. An output ram shaft position sensor, 29, provides a signal representing the linear position of that shaft as a feedback signal to controller 22. Thereby, controller 22 controls the fuel output flow rate of that pump through outlet 14 thereof. In addition, pressured fluid conduit 15 allows the pressurized fuel output of pump 11 to be supplied to electrohydraulic servovalve 22 to provide steady quiescent flow as well as provide transient flow to actuate pump transients.
Such an elaborate electrohydraulic control system for vane pumps requires quite a number of component parts some of which are relatively expensive, plus requires quiescent flow and transient flows, affecting dynamic fuel response to the gas turbine engine, as well as pump sizing, and has limitations in failure modes such as in its ability to provide fixed fuel flow following a failure. In addition, various failure modes are introduced in such a complex system with so many system components, and the working fluid used therein is subject to contamination leading to further possible failure modes. Thus, there is a desire for a less complex control for controlling the displacement of vane pumps especially those used as fuel pumps for gas turbine engines.